That's a tough one, because it begs the question, 'Best for what?' If I'm going alone, ready access to lots of good diving, with a straight-forward, low-hassle topside (food, bed, AC, a few scenic views to photograph) and everybody speaks English and accepts U.S. money & credit cards, prices aren't outrageous and total trip cost is good.
Bonaire by a land-slide. I was impressed with St. Croix, in that I could do 4 boat dives/day (can do that in Key Largo, too, and I think better diving), and there are shore diving options (which Key Largo mostly lacks). Haven't dove Little Cayman land-based (just live-aboard), but I think it checks the boxes. Never been to Roatan, but from the trip reports and descriptions provided by others, I think it'd make the cut. Left off Curacao because it has a rep. for diving much like Bonaire, but sounds more complicated getting site-to-site fast, and I don't want to pick both.
So I'd pick Bonaire, and expect Little Cayman and CocoView Resort (by reputation) in Roatan to be my 3. One big caveat; going to any of these, especially the latter 2, I'd go alone. I know CocoView's single supplement isn't ridiculous; Little Cayman's website wants me to submit info. rather than telling me rates upfront, which makes me nervous. Roatan's reputation for having to use repellent constantly outdoors due to 'sand fleas' is off-putting.
If I focus on topside, it's likely I'm bringing my wife, 5-year old daughter and maybe mother-in-law. Big game changer. Forget Bonaire (high airfare, limited topside activities), Little Cayman (ain't paying for non-divers, limited topside activities) or CocoView (they don't accept guests under 10).
Grand Cayman (probably Sunshine Suites Resort or Comfort Suites on 7 Mile Beach) would fill the bill (if wife hadn't picked getting an above-ground pool this year, I'd have tried for this). St. Croix worked fine. You said Caribbean, so I'm leaving out Florida options. I could maybe argue Cozumel or Belize, but I think I'd pick Cozumel, probably staying at an A.I. resort (sandy beach, kid's club, cost containment, not wandering around to eat, kid can run around without so much worry about traffic).
For family, I'd pick Grand Cayman, Cozumel, and St. Croix (but you could make arguments for Belize, St. Lucia, some other places).
Richard.